Oxygen concentration detecting apparatus

ABSTRACT

An oxygen concentration detecting apparatus precisely and easily performs diagnosis of a limit current type oxygen sensor. The limit current type oxygen sensor has an oxygen concentration detecting element for outputting limit current proportional to the oxygen concentration and a heater for heating the detecting element. A CPU of a microcomputer controls energization of the heater to activate the oxygen sensor. The CPU calculates element resistance based on the voltage applied to the oxygen sensor and the current detected in the sensor. In a sensor diagnosis routine, the CPU determines whether preconditions for the diagnosis have been met. If all the preconditions have been met, the CPU executes the diagnosis. That is, the CPU determines whether the element resistance is within a predetermined range. If it is below the range, the CPU determines that the sensor has high element temperature abnormality. If the element resistance is above the range, the CPU determines that the sensor has low element temperature abnormality.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims priority from JapanesePatent Application No. Hei. 7-76338, incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxygen concentration detectingapparatus having a limit current type oxygen sensor comprising an oxygenconcentration detecting element that outputs limit current proportionalto oxygen concentration and a heater for heating the detecting elementand, more particularly, to an oxygen concentration detecting apparatusthat checks for abnormality of the limit current type oxygen sensor.

2. Description of Related Art

Many modern air-fuel ratio control systems use limit current type oxygensensors (oxygen concentration detectors). In such a system, the oxygenconcentration detected by the air-fuel ratio sensor is inputted to amicrocomputer to calculate an air-fuel ratio, and the microcomputerperforms air-fuel ratio feedback control based on the calculatedair-fuel ratio. The control system thereby achieves optimal combustionin the internal combustion engine and reduces harmful substances inexhaust gas, such as CO, HC, NOx and the like.

However, since the control precision of the air-fuel ratio controlsystems is heavily degraded if the reliability of detection of theair-fuel ratio deteriorates, there has been a strong demand for atechnology that precisely detects an abnormality of an air-fuel ratiosensor. For example, Japanese Unexamined Patent Application PublicationNo. Hei. 1-232143, "Air-Fuel Ratio Control Apparatus for InternalCombustion Engine", describes a technology that detects an abnormalityof a heater if the temperature of the air-fuel ratio sensor (oxygenconcentration detecting element) detected by a temperature sensor failsto rise to a predetermined temperature. Japanese Unexamined PatentApplication Publication No. Hei. 3-189350, "Oxygen Sensor Heater ControlApparatus", describes a technology for use in an apparatus forcontrolling the power supply to the heater so that the heater resistancebecomes equal to a target resistance, the technology detecting anabnormality of the target resistance if the power supply to the heaterdeviates from a predetermined range.

However, the conventional art has the following problems. Theaforementioned former technology (Japanese Unexamined Patent ApplicationPublication No. Hei. 1-232143) requires a sensor for detecting thetemperature of the air-fuel ratio sensor, and thus has problems of highcosts. The latter technology (Japanese Unexamined Patent ApplicationPublication No. Hei. 3-189350) merely determines whether the targetresistance is properly set, and the occasions when this diagnosistechnology detects abnormality are substantially limited to theoccasions when the battery or the sensor has been replaced. Thus, thistechnology does not make a determination regarding the reliability ofthe oxygen sensor.

SUMMARY OF THE INVENTION

In view of the problems of the conventional art, an object of thepresent invention is to propose a novel diagnosis technology and therebyprovide an oxygen concentration detecting apparatus that precisely andeasily checks for abnormality of a limit current type oxygen sensor.

This object is achieved according to a first aspect of the presentinvention by providing an oxygen concentration detecting apparatus whichdetermines whether the oxygen sensor is abnormal on the basis of whetherthe element temperature of the oxygen sensor is within a predeterminedrange. Thereby, this apparatus precisely and easily performs the sensordiagnosis.

Preferably, the oxygen concentration detecting apparatus performs thesensor diagnosis to distinguish a low element temperature abnormalityand a high element temperature abnormality.

It is also possible that the oxygen concentration detecting apparatusdetermines whether the oxygen sensor is abnormal on the basis of whetherthe output from the oxygen sensor has changed within a predeterminedrange in response to an increase or a decrease of the fuel supply. Thus,this construction precisely and easily performs the sensor diagnosis.

Moreover, it is possible that the oxygen concentration detectingapparatus performs the sensor diagnosis when the oxygen sensor is ormust be activated, thus achieving accurate diagnosis.

Further, the system may feedback-control the heater power supply to makethe element temperature of the oxygen sensor substantially equal to atarget element temperature and perform the diagnosis of the oxygensensor on the basis of whether the heater power supply is greater than apredetermined abnormality determination criterion. Thus, this systemprecisely and easily performs the sensor diagnosis.

Also, the apparatus may achieve optimal diagnosis in accordance with theoperating conditions of the engine.

The apparatus may perform the diagnosis of the oxygen sensor on thebasis of whether the accumulation of the heater power supply is greaterthan a predetermined abnormality determination criterion. Thus, thisapparatus enhances the precision of diagnosis data and achieves accuratediagnosis.

Moreover, the apparatus may allow the sensor diagnosis to be executedonly-if the initial heater resistance is equal to or less than apredetermined value that indicates the cold state of the oxygen sensor.Thus, the apparatus inhibits the sensor diagnosis, for example, when theengine is restarted after warming up and the accumulation of the heaterpower supply is relatively small, thereby maintaining the high precisionof the sensor diagnosis.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and advantages of the present invention will be morereadily apparent from the following detailed description of preferredembodiments thereof when taken together with the accompanying drawingsin which:

FIG. 1 schematically illustrates the overall construction of an air-fuelratio control apparatus according to an embodiment of the invention;

FIG. 2 illustrates a sectional view of an oxygen sensor and the circuitconstruction of an ECU in the first embodiment;

FIG. 3 is a graph indicating the voltage-current characteristics of theoxygen sensor according to the first embodiment;

FIG. 4 is a flowchart illustrating a heater energization control routineof the first embodiment;

FIG. 5 is a graph indicating the relationship between the elementtemperature and the element resistance in the first embodiment;

FIG. 6 is a flowchart illustrating an air-fuel ratio detecting routineof the first embodiment;

FIG. 7 is a graph indicating the current-voltage characteristics of theoxygen sensor of the first embodiment;

FIG. 8 is a flowchart illustrating a sensor diagnosis routine of thefirst embodiment;

FIG. 9 is a flowchart illustrating a fail-safe routine of the firstembodiment;

FIG. 10 is a flowchart illustrating sensor diagnosis routine accordingto a second embodiment of the present invention;

FIGS. 11A-11C are graphs indicating the current-voltage characteristicsof the oxygen sensor, when the element is normal (FIG. 11A), when theelement temperature is abnormally low (FIG. 11B), and when the elementtemperature is abnormally high (FIG. 11C), respectively;

FIGS. 12A-12D are timing charts indicating the operation of heatercontrol according to a third embodiment of the present invention;

FIG. 13 is a flowchart illustrating a heater control routine accordingto the third embodiment;

FIG. 14 is a flowchart illustrating a processed data calculating routinein the third embodiment;

FIG. 15 is a flowchart illustrating a sensor diagnosis routine accordingto the third embodiment;

FIG. 16 is a flowchart illustrating a sensor diagnosis routine accordingto a fourth embodiment;

FIG. 17 illustrates a map for retrieving a heater power criterionaccording to the fourth embodiment;

FIGS. 18A-18E are timing charts indicating the operation of heatercontrol according to a fifth embodiment;

FIGS. 19,19A and 19B are a flowchart illustrating a heater controlroutine according to the fifth embodiment;

FIG. 20 is a graph indicating the relationship between the initialheater resistance and the accumulation of target power in the fifthembodiment; and

FIG. 21 is a flowchart illustrating a heater diagnosis routine accordingto the fifth embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

A first embodiment of the present invention wherein the oxygenconcentration detecting apparatus of the present invention is embodiedin an air-fuel ratio control apparatus of an automotive internalcombustion engine will be described with reference to the accompanyingdrawings.

FIG. 1 schematically illustrates the overall construction of theair-fuel ratio control apparatus of the internal combustion engineaccording to the first embodiment of the present invention. Referring toFIG. 1, a four-cylinder spark-ignition type gasoline internal combustionengine (hereinafter, referred to as "engine") 1 is connected to anintake pipe 2 and an exhaust pipe 3. An air cleaner 4 is provided in amost upstream portion of the intake pipe 2. A surge tank 5 is providednear the middle of the intake pipe 2. Disposed upstream from the surgetank 5 is a throttle valve 17 that is operated responsive to thedepression of an accelerator pedal (not shown). A bypass passage 18bypassing the throttle valve 1 is provided with an ISC valve (idle speedcontrol valve) 19.

The intake pipe (intake ports) 2 connected to each cylinder of theengine 1 is provided with an injector 6 thereat. Fuel is pumped from afuel tank 7 by a fuel pump 8, and then supplied to a pressure regulator10 via a fuel filter 9. The pressure regulator 10 supplies the injector6 with fuel with a regulated constant pressure, and also returns surplusfuel to the fuel tank 7. The injector 6 opens its valve to inject fuelby power supply from a battery 15. The fuel injected from the injector 6is mixed with intake air to form a fuel-air mixture. The mixture is thenintroduced into a combustion chamber 12 by an intake valve 11.

An intake air temperature sensor 20 is disposed near the air cleaner 4to detect the temperature of intake air. The surge tank 5 is providedwith an intake pipe pressure sensor 22 for detecting the pressure insidethe intake pipe 2 (intake negative pressure). The cylinder block of theengine 1 is provided with a coolant temperature sensor for detecting thetemperature of the engine coolant.

A spark plug 13 is disposed in the combustion chamber 12 of eachcylinder. An ignitor 14 generates a high voltage from the voltagesupplied from the battery 15. The high voltage is then distributed tothe spark plug 13 of each cylinder by a distributor 16. The distributor16 comprises a cylinder distinguishing sensor 24 and a crank anglesensor 25. The crank angle sensor 25 generates crank angle signals atpredetermined crank angles (for example, every 30° CA) during revolutionof the crankshaft of the engine 1. The cylinder distinguishing sensor 24generates cylinder distinguishing signals at a specific timing withrespect to a specific cylinder (for example, the compression TDC of thefirst cylinder) during revolution of the crankshaft of the engine 1.

The exhaust pipe 3 of the engine 1 is provided with a limit current typeoxygen sensor 26 that outputs detection signals linear with(proportional to) the oxygen concentration in exhaust gas. Disposeddownstream from the oxygen sensor 26 is a catalytic converter thatcleans exhaust gas.

The detection signals from the aforementioned sensors are inputted to anelectronic control unit (hereinafter, referred to as "ECU") 40. The ECU40 operates on the power supply from the battery 15. Upon receiving anON-signal from an ignition switch 28, the ECU 40 starts the engine 1.During operation of the engine 1, the ECU 40 feedback-controls theair-fuel ratio approximately to a target air-fuel ratio (for example,the theoretically optimal air-fuel ratio) by varying the air-fuel ratiocorrection coefficient on the basis of the signals from the oxygensensor 26. Furthermore, the ECU 40 performs sensor diagnosis operation(described later) to determine whether an abnormality has occurred inthe oxygen sensor 26, and when an abnormality has occurred, turns on awarning light 29 to inform the driver of the abnormality.

FIG. 2 shows a schematic sectional view of the oxygen sensor 26 and thecircuit construction of the ECU 40 connected to the oxygen sensor 26.The oxygen sensor 26 projects into the exhaust pipe 3, as shown in FIG.2, and comprises a cover 31, a sensor body 32 and a heater 33. The cover31 has a generally "U" sectional shape, and its peripheral wall has manypore connect the interior of the cover 31 and its exterior. The sensorbody 32 produces limit current corresponding to the oxygen concentrationin the lean region of the air-fuel ratio or the concentration of carbonmonoxide (CO) in the rich region of the air-fuel ratio.

The construction of the sensor body 32 will be described in detail. Anexhaust gas-side electrode layer 36 is fixed onto the outer surface of asolid electrolyte layer 34 having a sectional shape of a cup. The innersurface of the solid electrolyte layer 34 is fixed to theatmosphere-side electrode layer 37. A diffused resistor layer 35 hasbeen formed on the outside of the exhaust gas-side electrode layer 36 byplasma spraying. The solid electrolyte layer 34 is composed of an oxygenion-conductive oxide sintered material in which a stabilizer, such asCaO, MgO, Y₂ O₃ or Yb₂ O₃ is dissolved in ZrO₂, HfO₂, ThO₂, Bi₂ O₃ orthe like. The diffused resistor layer 35 is composed of a heat-resistantinorganic substance such as alumina, magnesia, quartzite, spinel, ormullite. The exhaust gas-side electrode layer 36 and the atmosphere-sideelectrode layer 37 are composed of a precious metal having highcatalytic activity, such as platinum, and are provided with a chemicallyplated porous coating. The exhaust gas-side electrode layer 36 has asurface area of about 10-100 mm² and a thickness of about 0.5-2.0 μm.The atmosphere-side electrode layer 37 has a surface area of 10 mm² orlarger and a thickness of about 0.5-2.0 μm. The solid electrolyte layer34 corresponds to the oxygen concentration detecting element in theappended claims.

The heater 33 is disposed in a space surrounded by the atmosphere-sideelectrode layer 37. The thermal energy from the heater 33 heats thesensor body 32 (the atmosphere-side electrode layer 37, the solidelectrolyte layer 34, the exhaust gas-side electrode layer 36 and thediffused resistor layer 35). The heater 33 has a sufficient heatgenerating capacity to activate the sensor body 32.

With this construction of the oxygen sensor 26, the sensor body 32generates a variable electromotive force at the point of the theoreticalair-fuel ratio, and produces limit current in accordance with the oxygenconcentration within the lean region defined with respect to thetheoretical air-fuel ratio. The limit current in accordance with theoxygen concentration varies depending on the area of the exhaustgas-side electrode layer 36, the thickness of the diffused resistorlayer 35, the porosity and the average pore size. The sensor body 32linearly detects the oxygen concentration. However, since a hightemperature of about 650° C. or higher is needed to activate the sensorbody 32 and the activation temperature range of the sensor body 32 isrelatively narrow, the thermal energy of exhaust gas from the engine 1is not sufficient to control the activation of the sensor body 32.According to this embodiment, the heater 33 is controlled as describedlater to achieve control of the temperature of the sensor body 32.Within a rich region with respect to the theoretical air-fuel ratio, onthe other hand, the concentration of carbon monoxide (CO), that is, anunburned gas, varies substantially linearly with the air-fuel ratio. Thesensor body 32 generates limit current in accordance with the COconcentration in the rich region.

The voltage-current characteristics of the sensor body 32 will bedescribed with reference to FIG. 3. The current-voltage characteristiccurves in FIG. 3 indicate that the current flowing into the solidelectrolyte layer 34 of the sensor body 32 in proportion to the oxygenconcentration (air-fuel ratio) detected by the oxygen sensor 26 islinear with the voltage applied to the solid electrolyte layer 34. Whenthe sensor body 32 is in the activated state at a temperature T=T1, thecurrent-voltage characteristics of the sensor body 32 exhibit a stablestate as indicated by characteristic curve L1 represented by solid linesin FIG. 3. The straight segments of the characteristic curve L1 parallelto the voltage axis V specify limit currents occurring in the sensorbody 32. The variation of the limit current parallels the variation ofthe air-fuel ratio (that is, lean or rich). More precisely, the limitcurrent increases as the air-fuel ratio shifts further to the lean side,and the limit current decreases as the air-fuel ratio shifts further tothe rich side.

The region of the voltage-current characteristic curve where the voltageis smaller than the levels corresponding to the straight segmentsparallel to the voltage axis V is a resistance-dominant region. Theslope of the characteristic curve L1 within such a resistance-dominantregion is determined by the internal resistance of the solid electrolytelayer 34 provided in the sensor body 32 (hereinafter, referred to as"element resistance"). The element resistance varies with temperature.As the temperature of the sensor body 32 decreases, the elementresistance increases and, therefore, the slope is reduced. When thetemperature T of the sensor body 32 is T2 which is lower than T1, thecurrent-voltage characteristics of the sensor body 32 become asindicated by the characteristic curve L2 represented by broken lines inFIG. 3. The straight segments of the characteristic curve L2 parallel tothe voltage axis V specify limit currents occurring in the sensor body32. The limit currents determined by the characteristic curve L2 aresubstantially equal to those determined by the curve L1.

With the characteristic curve L1, if a positive voltage is applied tothe solid electrolyte layer 34 of the sensor body 32, the currentflowing through the sensor body 32 becomes a limit current Ipos (seepoint Pa in FIG. 3). If a negative voltage is applied to the solidelectrolyte layer 34 of the sensor body 32, the current through thesensor body 32 becomes a negative limit current Ineg that is notdependent on the oxygen concentration but is instead proportional solelyto the temperature (see point Pb in FIG. 3).

Referring again to FIG. 2, the exhaust gas-side electrode layer 36 ofthe sensor body 32 is connected to a bias control circuit 41 that isconnected to the atmosphere-side electrode layer 37 of the sensor body32 via a positive bias DC power source 42. The bias control circuit 41is generally composed of the positive bias DC power source 42, anegative bias DC power source 43 and a change-over switch circuit 44.The negative electrode of the positive bias DC power source 42 and thepositive electrode of the negative bias DC power source 43 are connectedto the exhaust gas-side electrode layer 36.

The change-over switch circuit 44 selectively connects only the positiveelectrode of the positive bias DC power source 42 to a sensor currentdetecting circuit 45 when switched to a first select state. Whenswitched to a second select state, the change-over switch circuit 44connects only the negative electrode of the negative bias DC powersource 43 to the sensor current detecting circuit 45. That is, when thechange-over switch circuit 44 is in the first select state, the positivebias DC power source 42 positively biases the solid electrolyte layer 34of the sensor body 32 so that current flows through the solidelectrolyte layer 34 in the positive direction. On the other hand, whenthe change-over switch circuit 44 is in the second select state, thenegative bias DC power source 43 negatively biases the solid electrolytelayer 34 so that current flows through the solid electrolyte layer 34 inthe negative direction. The terminal voltages of the positive andnegative bias DC power sources 42, 43 correspond to the aforementionedapplied voltages Ipos, Ineg, respectively.

The sensor detecting circuit 45 detects the current flowing from theatmosphere-side electrode layer 37 of the sensor body 32 to the switchcircuit 44 or in the reverse direction, that is, the current flowingthrough the solid electrolyte layer 34. A heater control circuit 46duty-cycle controls the power supplied from a battery power source VB tothe heater 33 in accordance with the heater temperature and/or theelement temperature of the oxygen sensor 26, thus controlling theheating by the heater 33. The current flowing through the heater 33(hereinafter, referred to as "heater current Ih") is detected by acurrent detecting resistor 50.

An A/D converter 47 converts the current detected by the sensor currentdetecting circuit 45 (Ipos, Ineg indicated in FIG. 3), the heatercurrent Ih, and the voltage applied to the heater 33 (hereinafter,referred to as "heater voltage Vh") into digital signals, and outputsthe signals to a microprocessor 48. The microprocessor 48 comprises aCPU 48a for executing various operations and a memory 48b composed of aROM and a RAM. In accordance with predetermined computer programs, themicroprocessor 48 controls the bias control circuit 41, the heatercontrol circuit 46 and a fuel injection control apparatus (hereinafter,referred to as "EFI") 49. The EFI 49 receives various signals from theaforementioned sensors as engine information and thereby detects intakeair temperature Tam, intake negative pressure Pm, coolant temperatureThw, engine speed, NE, vehicle speed Vs and the like. Based on suchengine information, the EFI 49 controls the fuel injection performed bythe injector 6. According to this embodiment, the CPU 48a of themicrocomputer 48 constitutes heater control means, element resistancedetecting means, sensor diagnostic means, and heater power supplyestimating means as recited in the appended claims.

The operation of this embodiment will be described with reference to thecontrol programs executed by of the CPU 48a of the microcomputer 48.Described hereinafter are heater energization control, air-fuel ratiodetecting operation, and then sensor diagnosis operation.

The flowchart of FIG. 4 illustrates a heater energization controlroutine executed in a predetermined cycle by the CPU 48a. In step 101,the CPU 48a determines the control state of the heater 33 on the basisof heater control flags F1, F2. According to the first embodiment,following the turning-on of the ignition switch 28, the heater controlmode shifts to 100% duty control, first heater energization control, andthen .second heater energization control in that order. The heatercontrol flag F1=1 indicates that the first heater energization controlis being performed. The heater control flag F2=1 indicates that thesecond heater energization control is being performed.

In an initial period of the heater energization control, the heatercontrol flags F1, F2 have been cleared to "0"(initial value), andtherefore the CPU 48a proceeds to step 102 to execute the 100% dutycontrol. More specifically, the CPU 48a controls the heater controlcircuit 46 shown in FIG. 2 with 100% duty to fix the power supply to theheater 33 to the maximum value, thus rapidly heating the heater 33. Instep 103, the CPU 48a reads in the heater resistance RH calculated onthe basis of the heater voltage Vh and the heater current Ih (RH=Vh/Ih).The CPU 48a then determines in step 104 whether the heater resistance RHequals or exceeds 2Ω (whether RH ≧2Ω). If RH <2Ω, then the CPU 48aimmediately ends this routine. In this case, the 100% duty control iscontinued.

On the other hand, if step 104 determines that the heater resistance RH≧2Ω, the CPU 48a proceeds to step 105 to set the heater control flag F1to "1", and then proceeds to step 106 to execute the first heaterenergization control. In the first heater energization control, thecontrol duty for the heater 33 is determined by using a first map basedon the engine load (for example, the intake negative pressure Pm) andthe engine speed NE. The first map has been arranged such that theelement temperature of the oxygen sensor 26 will become a predeterminedactivating temperature; for example, a large control duty is set for alow-load or low-speed operational region since the thermal energy ofexhaust gas is small in such a region. Once the flag F1 has been thusset, the CPU 48a jumps from step 101 to step 106 to execute the firstheater energization control.

In step 107 following step 106, the CPU 48a reads in the elementresistance of the oxygen sensor 26 (the internal resistance of the solidelectrolyte layer 34) Zdc. The element resistance Zdc is calculated onthe basis of the element applied voltage Vneg (negative applied voltage)and the negative current Ineg detected by the sensor current detectingcircuit 45 (Zdc=Vneg/Ineg). In step 108, the CPU 48a determines whetherthe element resistance Zdc has become 90Ω or lower (whether Zdc ≦90Ω).If Zdc >90Ω, then the CPU 48a immediately ends the routine. In thiscase, the first heater energization control is continued. For reference,the relationship between the element temperature and the elementresistance Zdc is indicated in FIG. 5.

On the other hand, if step 108 determines that Zdc ≦90Ω, then the CPU48a proceeds to step 109 to set the flag F1 to "0" and the flag F2 to"1", and in step 110 executes the second heater energization control.The second heater energization control uses a second map, different fromthe first map, to determine a control duty for the heater 33 (ofgenerally the same characteristics as in the first heater energizationcontrol) in accordance with the engine load (for example, the intakenegative pressure Pm) and the engine speed NE. Once the flag F2=1 hasbeen set, the CPU 48a jumps from step 101 to step 110 to execute thesecond heater energization control. As described above, this embodimentopen-loop controls the energization of the heater 33 by the 100% dutycontrol in the initial period of the control operation, and then by thefirst energization control followed by the second heater energizationcontrol.

The flowchart of FIG. 6 illustrates an air-fuel ratio detecting routinestarted in response to the turning-on of the ignition switch 28 andexecuted by the CPU 48a in a cycle of, for example, 8 msec.

In steps 201-204 in FIG. 6, the CPU 48a executes procedures fordetermining activation of the sensor. Step 201 applies a predeterminedvoltage Vm within an element resistance detecting region indicated inFIG. 7 (for example, Vm=-1 volt). Step 202 reads in the current Im (seeFIG. 7) detected by the sensor current detecting circuit 45 shown inFIG. 2. Step 203 calculates an element resistance Zdc based on theapplied voltage Vm and the detected current Im (Zdc=Vm/Im).

In step 204, the CPU 48a determines whether the oxygen sensor 26 hasbeen activated on the basis of whether the element resistance Zdc iswithin a predetermined activation range (KREL-KREH). More specifically,if KREL ≦Zdc ≦KREH, that is, step 204 makes an affirmativedetermination, then it is determined that the oxygen sensor 26 has beenactivated. The CPU 48a then proceeds to step 205. On the other hand, ifstep 204 makes negative determination, the CPU 48a repeats steps 201-204until the sensor activation is determined.

In step 205, the CPU 48a applies 0.4 volt to the oxygen sensor 26 as theinitial value of the applied voltage Vp within an air-fuel ratiodetecting range indicated in FIG. 7. Then in step 206, the CPU 48a readsin the limit current Ip(n) detected by the sensor current detectingcircuit 45 shown in FIG. 2. The CPU 48a converts the limit current Ip(n)into an air-fuel ratio (A/F) in step 207. In step 208, the CPU 48acalculates an apply voltage Vp(n+1) for the next performance of theair-fuel ratio detection {Vp(+1)=f(Ip)}, and applies the apply voltageVp(n+1) to the oxygen sensor 26. Referring to FIG. 7, if the air-fuelratio is "16" in operation cycle (n) and "15" in operation cycle (n+1) ,application of Vp(n) results in detection of Ip(n) and then applicationof Vp(n+1) results in detection of Ip(n+1).

Then, CPU 48a determines in step 209 whether a predetermined length oftime has elapsed following the start of the air-fuel ratio detection. Ifthe predetermined length of time has not elapsed, the CPU 48a repeatssteps 206-209. If the predetermined length of time has elapsed, the CPU48a proceeds to step 210. In steps 210-213, the CPU 48a performs sensoractivation determining operation as in steps 201-204.

More specifically, the CPU 48a determines in step 213 whether theelement resistance Zdc determined through steps 210-212 is within thepredetermined activation range (KREL-KREH). If KREL ≦Zdc ≦KREH, then itis determined that the oxygen sensor 26 has been activated. The CPU 48athen proceeds to step 206. On the other hand, if step 213 makes negativedetermination, the CPU 48a repeats steps 210-213.

The sensor diagnosis routine will be described with reference to FIG. 8.The routine as illustrated by the flowchart of FIG. 8 is executed by theCPU 48a in a predetermined cycle of, for example, 32 msec. Through steps301-307 in FIG. 8, the CPU 48a determines whether preconditions for thesensor diagnosis have been established. More specifically, step 301determines whether the intake air temperature Tam equals or exceeds apredetermined criterion KTA (for example, 5° C.). Step 302 determineswhether the coolant temperature Thw equals or exceeds a predeterminedcriterion KTW (for example, 5° C.). Step 303 determines whether theengine speed NE equals or exceeds a predetermined criterion KNE (forexample, 500 rpm). Step 304 determines whether the vehicle speed Vs isless than a predetermined criterion KSPD (for example, 100 km/h). Step305 determines whether the elapsed time CAST following the start of theengine 1 equals or exceeds a predetermined criterion KCAST (for example,20 seconds). Step 305 determines whether the battery voltage VB equalsor exceeds a predetermined criterion KVB (for example, 13 V). Step 307determines whether a fuel cut flag XFC for indication of performance offuel-cut operation is cleared to "0", that is, whether the fuel cutoperation remains unperformed.

Of the aforementioned preconditions, the elapsed time CAST following thestart of the engine 1 and the battery voltage VB are used to estimate anaccumulated heater power supply. It is determined that the accumulationof heater power supply has reached or exceeded a predetermined valuewhen these values become equal to or greater than predetermined values.If these conditions have been established, it is assumed that the oxygensensor 26 has been activated or must be activated, and the CPU 48aallows the diagnosis to be performed. These preconditions for diagnosisprovide precise diagnosis.

If any of steps 301-307 makes a negative determination, the CPU 48aimmediately ends this routine. If all of steps 301-307 make affirmativedeterminations, the CPU 48a proceeds to step 308 to execute the sensordiagnosis based on the element resistance Zdc of the oxygen sensor 26.The element resistance Zdc of the oxygen sensor 26 is calculated as insteps 201-203 described above.

In step 308, the CPU 48a determines whether the element resistance Zdcis less than a first criterion KREL (10Ω according to this embodiment).If Zdc <KREL, the CPU 48a proceeds to step 309. The element resistanceZdc less than the first criterion KREL means that the elementtemperature has risen too high. In this case, the CPU 48a determinesthat the oxygen sensor 26 has a "high element temperature abnormality".The high element temperature abnormality includes the following modes: amode wherein the heater resistance of the oxygen sensor 26 varies tosmaller values to allow excessively large currents; and a mode whereinthe ground-side wire harness of the heater 33 is constantlyshort-circuited to ground so that the current control fails, thusallowing excessively large currents.

On the other hand, if Zdc ≧KREL, the CPU 48a determines in step 310whether the element resistance Zdc equals or exceeds the secondcriterion KREH (90Ω according to this embodiment). If Zdc ≧KREH, thenthe CPU 48a proceeds to step 311. The element resistance Zdc equaling orexceeding the second criterion KREH means that the element temperaturehas remained too low. Therefore, the CPU 48a determines in step 311 thatthe oxygen sensor 26 has a "low element temperature abnormality". Thelow element temperature abnormality includes the following modes: a modewherein the heater resistance of the oxygen sensor 26 varies to largevalues, thereby reducing the current; a mode wherein the heater 33deteriorates to increase its resistance, thereby reducing the current;and a mode wherein the wire harness of the heater 33 is disconnected,thus preventing the current from passing through the sensor.

If the aforementioned element abnormality of oxygen sensor 26 isdetermined, a fail-safe routine illustrated in FIG. 9 is performed (forexample, in a cycle of 32 msec). In step 401 in FIG. 9, the CPU 48adetermines whether the element abnormality has occurred. If the elementabnormality (high temperature abnormality or lower temperatureabnormality) has been determined in the operation illustrated in FIG. 8,the CPU 48a proceeds to step 402 to stop the air-fuel ratio feedback.Then, the CPU 48a discontinues the energization of the heater 33 in step403, and turns on the warning light 29 to indicate occurrence of theelement abnormality in step 404. The procedure of step 404 may bedesigned to indicate the high temperature abnormality and the lowtemperature abnormality in separate manners.

As described above, the first embodiment determines whether abnormalityhas occurred in the oxygen sensor 26 on the basis of whether the elementresistance of the oxygen sensor 26 is within the predetermined range(steps 308-311 in FIG. 8). More specifically, the output characteristicsof the limit current type oxygen sensor 26 are determined or specifiedby the slope of the characteristic curve within the resistance-dominantregion as shown in FIG. 3 (the slope of a segment of the curvecorresponding to voltages smaller than the voltages corresponding to thestraight segment of the curve parallel to the voltage axis), that is,the magnitude of the element resistance. If the oxygen sensor 26 isabnormal, the element resistance becomes too large or too small.Utilizing this phenomenon, abnormality of the oxygen sensor 26 can beprecisely and easily determined.

In addition, this embodiment determines whether the oxygen sensor 26 haslow element temperature abnormality (or high element temperatureabnormality) on the basis of whether the element resistance of theoxygen sensor 26 is above (or below) the allowed range. Morespecifically, if the element resistance is too high, it can bereasonably considered that the element temperature is too low, and thusthe low element temperature abnormality is determined. If the elementresistance is too low, it can be reasonably considered that the elementtemperature is too high, and thus the high element temperatureabnormality is determined.

Furthermore, since unlike the conventional art, this embodiment requiresno temperature sensor for detecting the element temperature, theembodiment will not suffer from a cost increase. Although a conventionaldevice can determine abnormality of the oxygen sensor mainly when thebattery or the sensor has been replaced, this embodiment constantlychecks for abnormality of the sensor during the traveling of thevehicle. Thus, this embodiment improves the reliability of the outputfrom the sensor and can provide a high-precision air-fuel ratio controlsystem.

Although the first embodiment performs the 100% duty control, the firstheater energization control and the second heater energization controlin that order, the method of heater energization control is not limitedby this embodiment. The other methods that may be employed are, forexample: a method in which only the first and second heater energizationcontrols are performed; and a method in which the 100% duty control isperformed for a predetermined length of time following the start of theengine, and then, for later operation, only the first and second heaterenergization controls are performed.

Second Embodiment

A second embodiment will be described mainly by referring to thefeatures distinguishing this embodiment from the first embodiment.According to the second embodiment, the CPU 48a provided in themicroprocessor 48 constitutes the heater control means, the fuel amountvarying means and the sensor diagnostic means in the appended claims.FIG. 10 shows a sensor diagnosis routine according to the secondembodiment.

In step 501 in FIG. 10, the CPU 48a determines whether preconditions forthe sensor diagnosis have been established. The determination regardingthe preconditions in step 501 corresponds to steps 301-307 in FIG. 8. Instep 502, the CPU 48a determines whether the air-fuel ratio feedback isbeing performed. If either step 501 or step 502 makes a negativedetermination, the CPU 48a ends this routine. If both step 501 and step502 make an affirmative determination, the CPU 48a proceeds to step 503.

In step 503, the CPU 48a stores the limit current Ip presently detectedby the sensor current detecting circuit shown in FIG. 2 as "Ipo". Instep 504, the CPU 48a stores the present engine operating conditions(the intake negative pressure Pm, the engine speed NE) as "Pmo" and"NEo".

Then, the CPU 48a increases or decreases the amount of fuel to beinjected by the injector 6 by α% (for example, 10%) in step 505, andthen determines in step 506 whether a predetermined length of time haselapsed following the fuel increase or decrease. The fuel increase meansthat the air-fuel ratio is forcibly shifted to the rich side, and thefuel decrease means that the air-fuel ratio is forcibly shifted to thelean side. When the predetermined length of time has elapsed followingthe fuel increase or decrease, the CPU 48a proceeds to step 507 todetermine whether the current intake negative pressure Pm and thecurrent engine speed NE substantially equal the values "Pmo" and "NEo"detected before the fuel increase (the values stored in step 504). Ifstep 507 determines that the engine operating conditions have changed,the CPU 48a immediately ends the routine without executing the sensordiagnosis. On the other hand, if step 507 determines that the engineoperating conditions have not changed, the CPU 48a proceeds to step 508to execute the sensor diagnosis.

In step 508, the CPU 48a reads in the limit current Ip presentlydetected by the sensor current detecting circuit 45. Then, step 509calculates a current change ΔIp between the current values before andafter the fuel increase (ΔIp=Ip-Ipo). The CPU 48a determines in step 510whether the current change ΔIp (absolute value) is greater than a firstcurrent criterion KDIL (whether ΔIp>KDIL). Step 511 determines whetherthe current change ΔIp (absolute value) is equal to or less than asecond current criterion KDIH (whether ΔIp ≦KDIH, where KDIL <KDIH). Theallowed range for current change (KDIL-KDIH) has been set correspondingto the actual change of the air-fuel ratio caused by the fuel increase.

If the current change ΔIp is within the range of KDIL-KDIH, CPU 48amakes affirmative determination in both step 510 and step 511. If ΔIp≦KDIL, CPU 48a makes negative determination in step 510 and thendetermines in step 512 that the low element temperature abnormality hasoccurred. If ΔIp>KDIH, the CPU 48a makes negative determination in step511 and then determines in step 513 that the high element temperatureabnormality has occurred.

FIGS. 11A, 11B and 11C are graphs indicating the signals outputted fromthe oxygen sensor 26 when the oxygen sensor 26 is normal, and when theoxygen sensor 26 has the low element temperature abnormality, and whenthe oxygen sensor 26 has the high element temperature abnormality,respectively. In the graphs, the current changes ΔIp1, ΔIp2, ΔIp3represent changes of the limit current caused by the changing of theapplied voltage from "Vp 1" to "Vp2". If the oxygen sensor 26 has thelow element temperature abnormality, the element resistance becomeslarge and the slope of the characteristic curve in theresistance-dominant region becomes small, as indicated in FIG. 11B.Thus, "66 Ip2" becomes smaller than "ΔIp1" that occurs in the normalconditions (ΔIp2 <ΔIp1). In this case, step 510 in FIG. 10 makes anegative determination, and thus the low element temperature abnormalityis determined. On the other hand, if the oxygen sensor 26 has the highelement temperature abnormality, the element resistance becomes smalland the slope of the curve in the resistance-dominant region becomesgreat, as indicated in FIG. 11C. Thus, "ΔIp3" becomes larger than "ΔIp1"that occurs in the normal conditions (ΔIp3>ΔIp1). In this case, step 511in FIG. 10 makes a negative determination, and thus the high elementtemperature abnormality is determined.

As described above, the second embodiment increases the fuel supply tothe engine 1 (in step 505 in FIG. 10), and determines whether the fuelincrease has caused a change of the output (limit current) from thesensor 26 within the predetermined range in order to determine whetherthe oxygen sensor 26 has an abnormality (steps 510-513 in FIG. 10). Withthis procedure, it can be determined whether the shift of the air-fuelratio to the rich side (decrease of the oxygen concentration) caused bythe fuel increase is properly reflected in the sensor output, so thatabnormality of the oxygen sensor 26 can be precisely and easilydetermined. In addition, since a criterion range is used fordetermination of abnormality, the embodiment is able to separatelydetermine the low element temperature abnormality and the high elementtemperature abnormality.

Third Embodiment

A third embodiment will be described. While the first and secondembodiments open-loop control the heater 33 of the oxygen sensor 26, thethird embodiment controls the heater 33 with feedback of the elementtemperature. According to this embodiment, the CPU 48a provided in themicroprocessor 48 constitutes the element resistance detecting means,the heater power supply control means and the sensor diagnostic means inthe appended claims.

FIGS. 12A-12D show timing charts indicating heater control according tothe third embodiment. More precisely, the timing charts indicate theoperation of the heater control performed from the starting ofenergization of the heater 33 in response to the starting of the engine1 until sufficient activation of the oxygen sensor 26. According to thisembodiment, the heater control can be divided into four modes (1)-(4) asindicated in FIGS. 12A-12D, in view of the different purposes andcontrol methods. These control modes will be described in sequence. Thecontrol modes (1)-(3) are performed to control the heater 33 before theoxygen sensor 26 is activated, and the control mode (4) is performed tocontrol the heater 33 after the oxygen sensor 26 has been activated.

In the control mode (1) performed immediately after the starting of theengine 1, the 100% duty heater voltage is applied to the heater 33(hereinafter, this control will be referred to as "full energizationcontrol"). That is, the maximum voltage is supplied to the heater 33 toquickly heat the heater 33 when the heater 33 and the sensor element(the sensor body 32) are cold.

The control modes (2) and (3) control the power supply to the heater 33to maintain the heater temperature at a target heater temperature (forexample, 1200° C.; that is, the upper limit heater temperature).Hereinafter, these control modes will be referred to as "power control".Since the heater temperature is specifically determined by the powersupply to the heater 33 if the element temperature is substantially theactivation temperature (700° C.), the temperature of the heater 33 canbe maintained at a constant level in such a case by continuing to supplya predetermined power. However, if the element temperature is low, thepower supply needed to maintain the heater temperature at a constantlevel varies with the element temperature. Normally, as the elementtemperature is lower, the power supply required is larger. During thepower control, the power supply to the heater 33 is controlled inaccordance with the element resistance (having the relationship with theelement temperature as indicated in FIG. 5).

However, in an initial period of the power control, the elementresistance is considerably large; that is, it exceeds the maximumdetectable value (for example, 600Ω). In such an element resistanceundetectable region, the power supply to the hater 33 is maintained at aconstant level (for example, 60 W) (control mode (2)). When the elementtemperature is increased so that the element resistance becomes 600Ω orlower, the power in accordance with the element resistance is thensupplied to the heater 33 (control mode (3)).

The control mode (4) feedback-controls the power supply to the heater 33to achieve an element resistance of 30Ω(corresponding to an elementtemperature of 700° C.) in order to maintain the activation of thesensor element (hereinafter, referred to as "element temperaturefeedback control").

A heater control routine according to the third embodiment will bedescribed with reference to FIG. 13.

In step 601 in FIG. 13, the CPU 48a determines whether the preconditionfor the element temperature feedback control have been established. Theprecondition is satisfied if the element resistance of the oxygen sensor26 is equal to or less than 30Ω. The CPU 48a determines in step 602whether the preconditions for the power control have been established.Two different preconditions have been arranged separately in accordancewith whether the oxygen sensor 26 (the sensor body 32 and the heater 33)is in a cold state or not. If the oxygen sensor 26 is in the cold state,the precondition is satisfied when a predetermined length of time haselapsed following the starting of the full energization control (thecontrol mode (1) indicated in FIGS. 12A-12D). If the oxygen sensor 26 isno longer in the cold state, the precondition is satisfied when theheater resistance has reached or exceeded a target heater resistance. Byexecuting the full energization control selectively when the oxygensensor 26 is in the cold state, an excessive rise of the heatertemperature can be prevented when the engine 1 is restarted.

If both step 601 and step 602 make a negative determination in aninitial period of the heater control, the CPU 48a proceeds to step 603to execute the full energization control of the heater 33 (the controlmode (1)). That is, the 100% duty heater voltage is applied to theheater 33.

If the preconditions for the power control are satisfied in step 602,the CPU 48a proceeds to step 604 to execute the power control (thecontrol modes (2), (3)). As described above, if the element resistanceis in the undetectable range (element resistance >600Ω), the powersupply to the heater 33 is controlled to a fixed value (the control mode(2)). If the element resistance becomes detectable, the power supply tothe heater 33 is controlled in accordance with the element resistance tomaintain the heater temperature to a target heater temperature (thecontrol mode (3)).

If the precondition for the element temperature feedback control issatisfied in step 601 in a later period, the CPU 48a proceeds to step605 to execute the element temperature feedback control (the controlmode (4)). For this control, the CPU 48a computes a heater control dutyDUTY based on equations (1)-(3):

    DUTY=DUTY.I+GP+GI+                                         (1)

    GP=KP·(Zdc-ZdcT)                                  (2)

    GI=GI+KI·(Zdc-ZdcT)                               (3)

where DUTY.I is an initial value of the control duty DUTY; ZdcT is acontrol target value (according to this embodiment, DUTY.I=20% andZdcT=30Ω; GP is a constant of proportionality; GI is an integral term;KP is a constant of proportionality; and KI is an integration constant(according to this embodiment, KP=4.2% and KI=0.2%). These values can beexperimentally determined, and will vary in accordance with thespecifications of the oxygen sensor 26.

The flowchart of FIG. 14 illustrating a processed data calculatingroutine executed by CPU 48a , for example, in a cycle of 128 ms. In step701 in FIG. 14, the CPU 48a reads in the heater current Ih detected bythe current detecting resistor 50 shown in FIG. 2. After reading in theheater voltage Vh in step 702, the CPU 48a calculates a heaterresistance RH by dividing the heater voltage Vh by the heater current Ih(RH=Vh/Ih) in step 703. Step 704 multiplies the heater voltage Vh by theheater current Ih to determine the heater power supply WH (WH=Vh-Ih).Then, the CPU 48a calculates a weighted average (hereinafter, referredto as "power average WLAV") of the heater power supply WH by anaveraging calculation {WHAV=(63-WHAVi-1+WH)/64}.

The flowchart of FIG. 15 illustrates a sensor diagnosis routine executedby the CPU 48a , for example, in a cycle of 1 second. The sensordiagnosis routine checks for sensor abnormality on the basis of theheater power supply WH needed during execution of the elementtemperature feedback control. More specifically, since the heater powersupply WH needed to maintain the element temperature at a target value(for example, 700° C.) increases if the oxygen sensor 26 hasabnormality, the sensor abnormality can be easily determined bycomparing that heater power supply WH with the normal value. Theprocedure of the diagnosis will be described with reference to FIG. 15.

In step 801 in FIG. 15, the CPU 48a determines whether a predeterminedlength of time KSTFB (for example, 10 seconds) has elapsed following thestart of the element temperature feedback control. Step 802 determineswhether a predetermined length of time KAFST (for example, 100 seconds)has elapsed following the last determination of abnormality. Further,step 803 determines whether a steady engine operating state (forexample, the idling state) has continued for a predetermined length oftime KSMST (for example, 5 seconds). If any of steps 801-803 makes anegative determination, the CPU 48a immediately ends this routine. Ifall of steps 801-803 make affirmative determinations, the CPU 48aproceeds to step 804.

The CPU 48a determines in step 804 whether the power average WHAV equalsor exceeds a predetermined heater power criterion KWHAV (whether WHAV≧KWHAV). If WHAV <KWHAV, it is considered that no sensor abnormality hasoccurred. The CPU 48a then proceeds to step 805 to clear an abnormalitydetermination flag XELER to "0", and then ends the routine.

On the other hand, if WHAV ≧KWHAV, then the CPU 48a proceeds to step 806to determine whether any abnormality other than sensor abnormality hasbeen detected. If no such abnormality has been detected, the CPU 48aproceeds to step 807 to determine whether the abnormality determinationflag XELER has been set to "1". If XELER=0, then the CPU 48a sets theabnormality determination flag XELER to 1in step 808. If XELER =1, theCPU 48a proceeds to step 809 to turn on the warning light to indicatethe occurrence of abnormality as a diagnosis indicating procedure. Inthe operation through steps 804-809, if occurrence of abnormality (WHAV≧KWHAV) is determined successively twice, the diagnosis indicatingprocedure is then executed.

As described above, the third embodiment feedback-controls the powersupply to the heater 33 so that the element resistance (elementtemperature) of the oxygen sensor 26 will become a target elementresistance 30106 (corresponding to an element temperature of 700° C.)(the element temperature feedback-control illustrated in FIG. 13), anddetermines whether the sensor 26 is abnormal on the basis of whether theheater power supply thus controlled is greater than a predeterminedabnormality determination criterion (steps 804-809 in FIG. 13). Sincethe element temperature feedback control will maintain the elementresistance (element temperature) within a desired activation range evenif sensor abnormality, such as sensor deterioration, occurs, aconsiderably large heater power supply is required if the oxygen sensor26 is abnormal. Utilizing this phenomenon, the third embodimentprecisely and easily detects sensor abnormalities. In addition, sincethe diagnosis operation is performed only during steady operation of theengine 1 (step 803 in FIG. 15), this embodiment avoids adverse effectsof the exhaust gas temperature on the heater power supply and thereforeperforms accurate diagnosis.

Fourth Embodiment

A fourth embodiment will be described. The fourth embodiment performsdiagnosis modified from the diagnosis according to the third embodiment.The flowchart of FIG. 16 illustrates a sensor diagnosis routineaccording to the fourth embodiment.

The routine illustrated in FIG. 16 executes step 820 in place of 803 inFIG. 15. Step 820 sets a heater power criterion KWHAV in accordance withthe engine operating conditions. The heater power criterion WHAV isdetermined by using a map shown in FIG. 17. That is, the criterion WHAVis determined (for example, to KWHAV1 or KWHAV2 as shown in FIG. 17) onthe basis of the current engine speed NE and engine load (intakenegative pressure Pm or intake air flow GN). The map has been arrangedso that the heater power criterion KWHAV decreases as the engine speedand/or the engine load increases, and so that the heater power criterionKWHAV increases as the engine speed and/or the engine load decreases.Thus, the fourth embodiment is able to perform optimal diagnosisoperation in accordance with the engine operating conditions.

Fifth Embodiment

A fifth embodiment will be described. According to this embodiment, theCPU 48a provided in the microcomputer 48 constitutes the poweraccumulation calculating means, the heater initial resistance detectingmeans, and the sensor diagnostic means.

The timing charts shown in FIGS. 18A-18E indicate heater controlaccording to the fifth embodiment. More precisely, the timing chartindicates the operation of the heater control performed following thestarting of energization of the heater 33 in response to the starting ofthe engine 1 until sufficient activation of the oxygen sensor 26.According to this embodiment, the heater control can be divided intofour modes (1)-(3) (that is, (1) full energization control, (2) powercontrol, and (3) element temperature feedback control) as indicated inFIGS. 18A-18E, in view of their different purposes and control methods.These control modes will be described in sequence.

In full energization control (the control mode (1)) performedimmediately after the starting of the engine 1, the 100% duty heatervoltage is applied to the heater 33. That is, the maximum voltage issupplied to the heater 33 to quickly heat the heater 33 when the heater33 and the sensor element are cold. The power control (the control modes(2)) controls the power supply to the heater 33 to maintain the heatertemperature at a target heater temperature (for example, 1200° C., thatis, the upper limit heater temperature). The element temperaturefeedback control (the control mode (3)) feedback-controls the powersupply to the heater 33 to achieve an element resistance of 30Ω(corresponding to an element temperature of 700° C.) in order tomaintain the activation of the sensor element. If the power supply tothe heater 33 exceeds an upper limit during the element temperaturefeedback control, the power supply to the heater 33 is regulated.

The flowchart shown in FIGS. 19A and 19B illustrates a heater controlroutine executed by the CPU 48a, for example, in a cycle of 128 ms. Theheater control and the diagnosis operation will be described withreference to this flowchart.

In step 901 in FIG. 19A, the CPU 48a determines whether the ignitionswitch 28 has been turned on (whether the power is on). If the power isoff, the CPU 48a ends the routine. If the power is on, the CPU 48aproceeds to step 902 to determine whether an initialization flag XINITis "0" (the initialization flag XINIT is initialized to "0" when thepower is switched on). If XINIT=0, the CPU 48a proceeds to step 903. IfXINIT=1, the CPU 48a proceeds to step 908.

Then, the CPU 48a stores the heater resistance RH determined on thebasis of the heater current Ih and the heater voltage Vh (RH=VH/Ih) asan initial heater resistance RHINT in step 903. Step 904 then determinesa target power accumulation WADTG based on the initial heater resistanceRHINT in accordance with the relationship indicated in FIG. 20. Step 905determines whether the initial heater resistance RHINT is equal to orless than a criterion KRHINT for determining a semi-activated state ofthe oxygen sensor 26. If RHINT ≦KRHINT, the CPU 48a sets a diagnosispermission flag XWADER to "1" in step 906.

Then, the CPU 48a sets the initialization flag XINIT to "1" in step 907and then proceeds to step 908. Once a target power accumulation WADTG isrequested and determined after the turning-on of the power, then step902 make negative determination and the operation will immediatelyproceed to step 908.

In step 908, the CPU 48a determines whether an element temperaturefeedback control flag XEFB is "1". In an initial period of the heatercontrol (prior to a time point t1 indicated in FIGS. 18A-18E), theelement temperature feedback control flag XEFB=0 and thus step 909 makesnegative determination. The CPU 48a then proceeds to step 909 todetermine whether the element resistance Zdc of the oxygen sensor 26 isequal to or less than 30Ω (corresponding to an element temperature of700° C.) corresponding to the temperature for performing the elementtemperature feedback control. If the element resistance Zdc is 30Ω orless, the CPU 48a proceeds to step 915. On the other hand, if theelement resistance Zdc is greater than 30Ω, the CPU 48a proceeds to step910.

The CPU 48a determines in step 910 whether the current heater resistanceRH equals or exceeds a learned heater resistance RHADP. The learnedheater resistance RHADP has been obtained by learning values of heaterresistance at a target heater temperature (for example, 1200° C.) usedfor the power control to eliminate the effect of variations of theheater resistance caused by individual product differences or changesover time. The CPU 48a determines in step 911 whether a poweraccumulation WADD equals or exceeds the target power accumulation WADTG(value determined in step 904). The power accumulation WADD isdetermined by a calculation routine (not shown), for example, bysuccessively accumulating a heater power supply WH (=Vh·Ih) detectedevery 128 ms (WADD=WADDi-1+WH).

If either step 910 or step 922 makes a negative determination (that is,RH <RHADP, or WADD <WADTG), the CPU 48a proceeds to step 912 to executethe full energization control (the control mode (1)). In the initialperiod prior to the time point t1 indicated in FIGS. 18A-18E, the CPU48a proceeds through steps 908, 909, 910, (911) and 912 in that order,to apply the 100% duty heater voltage to the heater 33.

If both step 910 and step 911 make affirmative determination (that is,RH ≧RHADP, and WADD ≧WADTG), the CPU 48a proceeds to step 920 to executethe power control (the control mode (2)). In the period t1-t2 indicatedin FIGS. 18A-18E, the CPU 48 proceeds through steps 908, 909, 910, 911and 920 in that order, to control the power supply to the heater 33 inaccordance with the element resistance to maintain the heatertemperature to a target heater temperature. In step 920, a power controlexecution flag XEWAT is set to "1".

At the time point t2 indicated in FIGS. 18A-18E, the CPU 48a makes anaffirmative determination in step 909, and then proceeds to step 915 todetermine whether the power control execution flag XEWAT is "1". IfXEWAT=1, the CPU 48a proceeds to step 930 to execute the learning ofheater resistance, and then proceeds to step 940. On the other hand, ifXEWAT=0, the CPU 48a immediately proceeds to step 940. The heaterresistance learning in step 930 determines whether the current heaterresistance RH is greater than a value obtained by the followingcalculation: the heater resistance learned value RHADP+α% (for example,α=2%). If the current heater resistance RH is greater than that value,the heater resistance learned value RHADP is updated to the currentheater resistance RH.

Then, the CPU 48a executes the heater diagnosis routine (describedlater) in step 940, and the element temperature feedback control in step950. In this case, the CPU 48a resets the power control execution flagXEWAT to "0" and sets the element temperature feedback control XEFB to"1". The CPU 48a determines the control duty DUTY for the heater controlcircuit 46 separately in three different manners (a) to (c) as follows.

(a) When the elapsed time following the turning-on of the power is apredetermined length of time (for example, 24.5 seconds) or longer, thecontrol duty DUTY is determined on the basis of equations (4)-(7):

    DUTY=GP+GI/16+GD                                           (4)

    GP=KP=·(Zdc-ZdcT)                                 (5)

    GI=GIi-1+KI·(Zdc-ZdcT)                            (6)

    GD=KD·(Zdci-Zdci-1)                               (7)

where ZdcT is a control target value (according to this embodiment,DUTY.I=20% and ZdcT=30Ω); GP is a constant of proportionality; GI is anintegral term; GD is a differential term; KP is a constant ofproportionality; KI is a constant of integration; and KD is adifferentiation constant.

(b) If the elapsed time following the turning-on of the power is lessthan the predetermined length of time (for example, 24.5 seconds) andthe air-fuel ratio >12, the control duty DUTY is calculated on the basisof equation (8) using the proportional term GP and the integral term GI:

    DUTY=GP+GI/16+GD                                           (8)

If the elapsed time following the turning-on of the power is less thanthe predetermined length of time (for example, 24.5 seconds) and theair-fuel ratio ≦12, the control duty DUTY is calculated on the basis ofequation (9). However, in this case (air-fuel ratio ≦12), the elementtemperature feedback control by PID is difficult and, therefore, theheater resistance feedback control is performed instead of the elementtemperature feedback control.

    DUTY=HDUTYi-1+KPA·(RHG-RH)                        (9)

where KPA is a constant and RHG is a target heater resistance (2.1Ω,corresponding to 1020° C.).

The heater diagnosis routine in step 940 in FIG. 19B will be describedwith reference to FIG. 21.

In step 941, the CPU 48a determines whether the diagnosis permissionflag XWADER is "1". If XWADER=0, the CPU 48a immediately ends theroutine. If XWADER=1, the CPU 48a proceeds to step 942 to determineswhether the power accumulation WADD equals or exceeds a predeterminedabnormality determination criterion KWADER (whether WADD ≧KWADER). IfWADD <KWADER, the CPU 48a proceeds to step 943 to clear an abnormalitydetermination flag XELER to "0".

On the other hand, if WADD ≧KWADER, the CPU 48a proceeds to step 944 todetermine whether the abnormality determination flag XELER has been setto "1". In the operation through steps 944-946, if the occurrence of anabnormality is determined successively twice, the diagnosis indicatingprocedure is then executed (the warning light 29 is turned on).

As described above, the fifth embodiment calculates accumulation (poweraccuanulation WADD) of the heater power supply from the start ofenergization of the heater 33, and determines whether the poweraccumulation WADD is greater than the predetermined abnormalitydetermination criterion KWADER to determine whether the oxygen sensor 26is abnormal (steps 942-946 in FIG. 21). By performing diagnosis based onthe accumulation of the heater power supply, this embodiment enhancesthe precision of diagnosis data and thereby provides accurate diagnosis.

Moreover, the fifth embodiment detects the initial heater resistance atthe start of energization of the heater 33 (step 903 in FIG. 19A) andallows the sensor diagnosis to be executed only if the initial heaterresistance is within a predetermined range such that it will bedetermined that the oxygen sensor 26 is in a cold state (that is, Yes instep 905 in FIG. 19A). For example, when the heater energization isstarted in response to the restart of the engine after warming-up, theaccumulation of the heater supply power is relatively small and it isnot preferable to use this accumulation as a basis for the diagnosis,considering the precision of the sensor diagnosis. Therefore, thisembodiment performs the diagnosis only when the oxygen sensor is in thecold state, and thus constantly provides good diagnosis.

Although the present invention has been fully described in connectionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications willbecome apparent to those skilled in the art. Such changes andmodifications are to be understood as being included within the scope ofthe present invention as defined by the appended claims.

What is claimed is:
 1. An oxygen concentration detecting apparatuscomprising:an oxygen sensor having an oxygen concentration detectingelement element resistance detecting means for detecting an elementresistance of said detecting element; and sensor diagnostic means forperforming diagnosis of said oxygen sensor in accordance with whethersaid element resistance of said detecting element detected by saidelement resistance detecting means is within a predetermined rangedefined by first and second nonzero, finite values.
 2. An oxygenconcentration detection apparatus according to claim 1, furthercomprising:a heater for heating said detecting element; and heatercontrol means for controlling an amount of energization of said heaterto activate said oxygen sensor in accordance with operating conditionsof an internal combustion engine.
 3. An oxygen concentration detectingapparatus according to claim 2, wherein said operating conditions ofsaid internal combustion engine include load and speed.
 4. An oxygenconcentration detecting apparatus according to claim 1, furthercomprising:a heater for heating said detecting element; and heater powersupply estimating means for estimating an accumulation of heater powersupply from starting of energization of said heater; wherein said sensordiagnostic means is for performing said diagnosis of said oxygen sensorafter it is determined that said accumulation of said heater powersupply estimated by said heater power supply estimating means is atleast a predetermined value.
 5. An oxygen concentration detectingapparatus according to claim 4, wherein said heater power supplyestimating means estimates said accumulation of said heater power supplybased on elapsed time following starting of an internal combustionengine.
 6. An oxygen concentration detecting apparatus comprising:alimit current type oxygen sensor having an oxygen concentrationdetecting element for outputting a limit current proportional to oxygenconcentration, and a heater for heating said detecting element; heatercontrol means for controlling energization of said heater to activateand to hold said element resistance of said oxygen sensor; elementresistance detecting means for performing diagnosis of said oxygensensor in accordance with whether said element resistance of saiddetecting element detected by said element resistance detecting means iswithin a predetermined range defined by first and second nonzero, finitevalues.
 7. An oxygen concentration detecting apparatus according toclaim 6, wherein said sensor diagnostic means performs diagnosis of saidoxygen sensor after oxygen sensor activated conditions are established.8. An oxygen concentration detecting apparatus according to claim 6,wherein said heater control means an amount of energization of saidheater in accordance with operating conditions of an internal combustionengine.
 9. An oxygen concentration detecting apparatus according toclaim 6, wherein said sensor diagnostic means detects said oxygen sensorhas high element temperature abnormality of said element resistance isless than a first criterion and low element temperature abnormality ifsaid element resistance equals or exceeds a second criterion.